This invention relates generally to signal monitoring devices for optical telecommunication networks, capable of detecting and measuring at least the power, or intensity, and preferably optical signal-to-noise ratio of each of a plurality of WDM channels transmitted over an optical waveguide, and particularly to such device employing a light deflector, a linear variable filter (LVF) and a photodetector.
One of the functions of a known optical performance monitor (OPM) is to identify and measure the power in each channel of a wavelength division multiplexed (WDM) signal. Some OPMs, as for example OPM512, a 50 GHz, 512 pixel monitor available from Ocean Optics, USA, are also capable of measuring the optical signal-to-noise ratio (OSNR) of multiple wavelength channels. OPMs without such capability are sometimes referred to as optical channel monitors (OCM).
U.S. Pat. Nos. 6,396,603 (Samsung Electronics) and 6,441,933 (LG Electronics) describe two exemplary devices for monitoring the performance of optical channels in telecommunication networks.
U.S. Pat. No. 6,075,647 (Hewlett-Packard) describes an optical spectrum analyzer including a polarization modifier that serves to eliminate polarization dependence of the input signal to be analyzed. The modified signal is then directed to a tiltable interference filter.
U.S. Pat. No. 6,118,530 discloses an optical scanning spectrometer utilizing a fixed fiber F-P filter in combination with a Fabry-Perot grating.
It is an object of the present invention to provide a compact optical performance monitoring device. It is also an object of the invention to provide such device that is durable, tunable, offers low loss, good wavelength registrability and spectral resolution.
In accordance with one aspect of the invention, there is provided an optical performance monitoring device for monitoring and analyzing the spectral properties of an optical signal in an input optical beam, the device comprising:
an optical deflecting means having a periodically movable reflective mirror coupled to receive the input optical beam and to deflect the beam in a one-dimensional direction to produce a deflected beam at a varying deflection angle,
a photodetector disposed to receive the deflected beam from the scanning means, and
a linear variable filter (LVF) disposed in the optical path of the deflected beam to pass a selected wavelength channel of the deflected beam in dependence upon the deflection angle.
The device may also comprise a focusing lens disposed to receive the deflected beam from the deflecting means and to direct the beam to the LVF at an approximately normal incidence angle. Advantageously, the focusing lens may also bring the deflected beam into a focus at the LVF.
In an embodiment of the invention, the LVF is fixed and the deflected light beam is moved relative thereto so as to be incident on various spots of the filter as the beam is deflected by the movable mirror.
It is also conceivable to move the LVF relative to a non-deflected optical beam in the absence of a movable mirror (as described in a co-pending U.S. patent application Ser. No. 10/059,413, the specification of which being incorporated by reference therewith), but such solution, requiring relatively fast filter displacement and control means, is somewhat inferior to the solution proposed by the present invention.